Determining the position, orientation or mere presence of a person within a defined space is important in applications ranging from medical treatment to safety and security. For example, prior to initiating operation of a tomographic scanning device, it is essential to ensure that the patient is not only within the device, but also oriented properly with respect to the scanning elements. While direct visual monitoring by trained personnel is obviously ideal, the nature of the equipment may make this impossible.
Of even greater concern are devices that operate automatically but rely on an assumption of proper orientation. For example, airbags are now employed as standard equipment on new automobiles, and have substantially reduced accident-related injury by cushioning the driver and passenger. However, because of their explosive deployment, airbags can themselves cause injury or even death of an infant improperly oriented in the passenger seat. They are also expensive to repack, so that passenger-side deployment into an empty seat is wasteful. Thus, although visual monitoring is simple and unimpeded in an automobile, it is not necessarily reliable; consumers may be unaware of proper airbag operation, and the routine character of operating an automobile invites inattentiveness: a driver might easily fail to disable the airbag after his passenger departs, or may orient a child improperly.
For such applications, where proper orientation is critical but difficult to monitor visually with adequate reliability, sensor arrays have been developed to obviate the need for human attention. These arrays typically contain a large number of sensing devices arranged to surround the space of interest; the sensors are often utilized in a binary fashion--detecting or not detecting presence within an operating range--so that numerous devices typically are necessary to resolve the different orientations of interest with a tolerable level of ambiguity. But the number of sensors is not chosen in any systematic or rigorous fashion; there is no methodology for minimizing the number of sensors, nor any test (other than experimentation) to ensure that the number chosen will resolve all necessary cases.
In order to ensure reliability, current systems may also require multiple sensing modalities (e.g., some combination of infrared, ultrasound, force and capacitance sensing). Once again, the reason for this stems from a "brute force" approach to distinguishing ambiguous cases.
For example, a common type of binary electrostatic sensor is a capacitive button switch, which is activated when the user places a finger thereon; in so doing, the user effectively increases the capacitance of a capacitor, with the resulting increase in capacitive current indicating actuation of the button. A more advanced version of this type of sensor is described in published PCT application WO 90/16045 (Tait), which describes an array of receiver electrodes arranged about a central transmitting electrode. Even this type of configuration, however, is only slightly more advanced than a purely binary system, since what is measured is variation in weighting among the arrayed electrodes. Arrangements such as this do not meaningfully reduce the number of devices (i.e., electrodes) necessary to characterize position and orientation, nor provide an approach to obtaining an optimum number of devices. Moreover, the Tait device is not employed in a manner that is even capable of resolving a three-dimensional mass distribution, much less distinguishing among different orientation/position cases. It is expected that contact will be complete in all cases--that is, the user's finger will actually touch the transmitting and receiving electrodes--rendering the approach unsuitable where such contact is not possible.